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Wednesday, March 19, 2008

Stone Mountain Natural History

Natural History

Stone Mountain sits on the western edge of a large belt of Lithonia Gneiss granite although the younger intrusive granite that comprises the mountain is entirely different from Lithonia granite. Rising to a height of 1,683 feet above sea level (roughly 650-750 feet above the surrounding Georgia Piedmont, depending on where it is measured), it is visible from Kennesaw Mountain to the west, Amicalola Falls State Park to the north and Mount Yonah to the northeast.

Technically known as a pluton, the mountain was formed during the complex folding and faulting that created the Blue Ridge Mountains, although Stone Mountain is not part of that range. The magma that created Stone Mountain was formed deep inside the earth, then forced its way out of the earth's molten center. Before the molten rock hit the air it stopped, initially forming the west side of the "pluton." Successive attempts at eruption (breaking through to the surface of the earth) also failed, but added to the size of the dome from west to east. Once the pluton was formed it began to cool. This occurred during the Alleghenian Orogeny, a massive collision of tectonic plates perhaps 350 million years ago.

Geologists, however, are stumped as to how this enormous rock (the largest known granite formation) became exposed. Some believe that the entire Georgia Piedmont region was higher than the mountain and over time, erosion simply wore the dirt and metamorphic stone surrounding the mountain away. Others believe that the area was flooded after the formation of the mountain and the water eroded the surrounding material. A third theory includes post-formation geological events (primarily earthquakes).

Analysis of the rock reveals that the magma that created the pluton was comprised of quartz, feldspar, microcline and muscovite with smaller amounts of biotite and tourmaline. Embedded in the granite are pieces of biotite gneiss and amphibolite

There are many ways that rock changes over time. The two most commonly associated with Stone Mountain are exfoliation and erosion. Granite is normally exfoliated in sheets, along the same natural lines that it is quarried. These lines, known technically as joints, run throughout Stone Mountain. They are expanded the erosive forces of heat, cold, weather (mostly wind and rain), gravity and plant life or any combination of these elements. Once a sheet on the surface as been exfoliated and the sheet of rock beneath it is exposed the process begins again.

The same erosive elements that open the joints in the rock work on the surface as well, but at a much slower rate. Erosion, for example, creates the vernal pools which form on level surfaces of the mountain. These pools are the indentations up to a few feet wide that fill with water during the spring (vernal is another word for spring). Many will remain damp or filled with water well into the summer, especially larger pools. A little shade from a scrub pine helps the pools hold the water long as well. Vernal pools are one type of weathering pits on the mountain. Watch for larger indentations, several feet wide, where trees and schrubs have taken hold and grow, adding their pressure to the eroding mountaintop.

Vernal pools play an important role in life at the top of the seemingly barren mountain. Just as in a desert, life at the top of Stone Mountains depends on the water available in the vernal pools. When the summer is wet the pools, ranging in size from several inches to several feet, may hold water most of the year. During drought years the pools can be dry from June until October. While larger mammals like squirrel and fox rely on the water for drinking other flora and fauna depend on the water for life.

Two types of shrimp frequently inhabit these pools, fairy shrimp and clam shrimp, as well as a unique variety of red moss.

Water that falls on the mountain that is not captured in these pools also plays a role in the evolution of the mountain. On the sides of the mountain lichen and moss come to life after a rain, but create a treacherous environment for hikers, especially on the Mountaintop Trail. As the water cascades down the sides of Stone Mountain it forms small streams near the base, creating a cooler, more diverse forested environment.

Stone Mountain's slopes offer a haven for a variety of birds (including migratory species in the Spring and Fall) and various small animals. Among the common birds are Hoot Owl, Red-tailed Hawk, Wood Ducks, Mallard, Great Blue Herons and various egrets. One overlooked species is the peregrine falcon, sometimes called a duck hawk, now off the endangered species list and an occasional visitor to the mountain. Mammals that inhabit the mountain include red fox, bobcat, grey squirrel, rabbits and deer.

Tucked in the crevices and outcroppings of rock that make up the majority of Stone Mountain's surface, a wide range of plant life makes its home. Most famous is the Stone Mountain Yellow Daisy (also known as the Confederate Daisy), found by Reverend Thomas Porter in 1846. The plant lends its name to the Yellow Daisy Festival, one of the most popular events in the Southeast United States.

Flowering yucca is also found on the mountain, as well as other flora including various species of pine tree, especially stunted loblolly, cactus, and a few small hardwoods.

Source: Larry Worthy, Editor-in-chief exclusively for About North Georgia